1. Field of the Invention
The present invention relates to hydraulic circuits for operating members of a machine, and more particularly to hydraulic circuits in which multiple actuators are powered in unison to operate a member.
2. Description of the Related Art
Construction and agricultural equipment have moveable members which are operated by actuators, such as hydraulic cylinder and piston arrangements, controlled by hydraulic valves. There is a present trend away from manually operated hydraulic valves in such equipment toward electrical controls and the use of solenoid valves. This type of control simplifies the hydraulic plumbing as the control valves do not have to be located in the operator cab with individual hydraulic lines extending to the actuators located throughout the equipment. The control valves can be located at the actuators with only hydraulic supply and return lines being run throughout the equipment. This change in technology also facilitates control of various machine functions by a computer.
Application of pressurized hydraulic fluid from a pump to the actuator often is controlled by a set of four proportional solenoid valves, such as described in U.S. Pat. No. 5,878,647. When an operator desires to move a member on the equipment, a control lever is operated to generate electrical signals that drive the solenoid valves for the cylinder associated with that member. One solenoid valve is opened to supply pressurized fluid to a cylinder chamber on one side of the piston and another solenoid valve opens to allow fluid to drain from a chamber on the other side of the piston. By varying the degree to which the solenoid valves are opened, the flow of fluid to or from the associated cylinder chamber is metered, thereby controlling that rate of piston movement. One pair of the valves in each set is used to move the actuator and the associated machine member in one direction, and the other valve pair produces movement in the opposite direction.
Machine members that move relatively heavy loads typically are operated by multiple actuators which function in parallel. For example, the boom of a front end loader has a pair of arms each raised and lowered by a separate piston-cylinder arrangement. Thus the load is split between two actuators and the mechanical assembly is referred to as a xe2x80x9csplit actuator mechanismxe2x80x9d or in the case of the front end loader a xe2x80x9csplit cylinder mechanism.xe2x80x9d The two cylinders were often controlled by a single control valve assembly connected to the cylinders by hoses. A safety valve had to be provided at each cylinder to prevent the boom from dropping in the event a hose burst. Alternatively, separate sets of four proportional solenoid valves were located at each cylinder and connected thereto by rigid tubing. If a hose bursts in this configuration, the valves could be closed to prevent the boom from dropping. However, this alternative required twice as many control valves in comparison to a single cylinder function and the associated restrictions.
Therefore, a desire exists to reduce the number of hydraulic valves that operate a split cylinder mechanism, while maintaining safe control of the mechanical members of the equipment.
A hydraulic system is provided to operate first and second actuators, such as the split cylinders of a front end loader, for example. Each of those actuators has first and second ports. The hydraulic system includes a primary control valve that has one port for connection to a source of pressurized hydraulic fluid, another port for connection to a tank for the hydraulic fluid, and a common port. A first control valve selectively connects the common port of the primary control valve to the first port of the first actuator. A second control valve is connected between the common port of the primary control valve and the first port of the second actuator. A third control valve selectively couples both the second port of the first actuator and the second port of the second actuator to the source of pressurized hydraulic fluid. A fourth control valve selectively connects both the second port of the first actuator and the second port of the second actuator to the tank for hydraulic fluid.
To operate the first and second actuators in one direction, the primary control valve is positioned to connect the source of pressurized hydraulic fluid to the common port and the fourth control valve is opened to form a fluid path between the second ports of both the first and second actuators and the tank. The first and second electrohydraulic proportional valves are operated to meter hydraulic fluid into the first and second actuators to control the rate of movement. The degree to which the fourth control valve is opened meters the flow of hydraulic fluid from the actuators.
To operate the first and second actuators in another direction, the primary control valve is positioned to connect the tank to the common port, and the third control valve is opened to form a fluid path between the second ports of both the first and second actuators and the source of pressurized hydraulic fluid. The degree to which the third control valve is opened meter the flow of hydraulic fluid to the first and second actuators, while first and second electrohydraulic proportional valves are operated to meter hydraulic fluid from those actuators.